Add initial riscv64 support

Notes:
- No bootloader is configured; images must be booted via direct kernel
  - Tested via qemu + opensbi fw_jump.elf + kernel/initrd
- As riscv64 is currently in ports, the following options are required:
    --distribution unstable
    --mirror-bootstrap http://ftp.ports.debian.org/debian-ports/
    --security false
    --debootstrap-options --keyring=/usr/share/keyrings/debian-ports-archive-keyring.gpg
- Normally ports require stacking two dists, unstable and unreleased.
  debootstrap does not support doing this, but as of this writing, there
  is nothing in riscv64 unreleased which is required to bootstrap.
  However, this would not have been possible a year ago.

functions/configuration.sh

@@ -251,6 +251,10 @@ Prepare_config ()
 			LB_LINUX_FLAVOURS_WITH_ARCH="${LB_LINUX_FLAVOURS_WITH_ARCH:-powerpc64le}"
 			;;
 
+		riscv64)
+			LB_LINUX_FLAVOURS_WITH_ARCH="${LB_LINUX_FLAVOURS_WITH_ARCH:-riscv64}"
+			;;
+
 		s390x)
 			LB_LINUX_FLAVOURS_WITH_ARCH="${LB_LINUX_FLAVOURS_WITH_ARCH:-s390x}"
 			;;

scripts/build/binary_linux-image

@@ -48,7 +48,7 @@ case "${LB_INITRAMFS}" in
 esac
 
 case "${LB_ARCHITECTURE}" in
-	powerpc|ppc64el)
+	powerpc|ppc64el|riscv64)
 		LINUX="vmlinux"
 		;;
 

scripts/build/binary_rootfs

@@ -37,7 +37,7 @@ case "${LB_ARCHITECTURE}" in
 		LINUX="vmlinuz"
 		;;
 
-	powerpc|ppc64el)
+	powerpc|ppc64el|riscv64)
 		LINUX="vmlinux"
 		;;
 esac